﻿Collective photothermal bending of flexible organic crystals modified with MXene-polymer multilayers as optical waveguide arrays

The performance of any engineering material is naturally limited by its structure, and while each material suffers from one or multiple shortcomings when considered for a particular application, these can be potentially circumvented by hybridization with other materials. By combining organic crystals with MXenes as thermal absorbers and charged polymers as adhesive counter-ionic components, we propose a simple access to flexible hybrid organic crystal materials that have the ability to mechanically respond to infrared light. The ensuing hybrid organic crystals are durable, respond fast, and can be cycled between straight and deformed state repeatedly without fatigue. The point of flexure and the curvature of the crystals can be precisely controlled by modulating the position, duration, and power of thermal excitation, and this control can be extended from individual hybrid crystals to motion of ordered two-dimensional arrays of such crystals. We also demonstrate that excitation can be achieved over very long distances (>3 m). The ability to control the shape with infrared light adds to the versatility in the anticipated applications of organic crystals, most immediately in their application as thermally controllable flexible optical waveguides for signal transmission in flexible organic electronics.

flexible optical waveguide is a novel subject. This work provides a simple and effective way to realize the flexibility of optical waveguide on organic crystals, and this method is applicable to a variety of materials. Moreover, 2D array with high sensitivity, high and controllable degree of deformation and durability over prolonged actuation is also carried out, while the bending point of each crystal can be controlled by changing the infrared light position. Therefore I recommend this article can be considered for publication after the authors have addressed the following concerns: 1)The bending of organic crystals caused by thermal effect should be further analyzed theoretically. Does it involve physical changes such as phase transition or chemical changes? 2)Does the sample size or deposition thickness affect the performance of the 2D array?
3)The authors state that "the organic crystals have the advantages of having defect-free structures", more characterization is needed to support this claim. 4)When it comes to fatigue and cycling, 100 times bending test is not enough. More cycles should be shown. 5)There are some typos and mistakes: the font should be uniform in Line 51, redundant words in Line 109, etc.
Reviewer #4 (Remarks to the Author): The manuscript describes the realization of rod-shaped organic crystals that exhibit a deformation response to IR light. This photoresponse is enabled by coating the crystals in a multilayer structure containing MXenes that absorb the IR irradiation. This absorption produces a local thermal response that triggers the actuation mechanism of the organic crystals. The work demonstrates this effect using a few different chemistries of organic crystals. The manuscript also shows that the location of the deformation can be controlled through the location of the illumination.
The manuscript is convincing in its conclusion and has a clear message. I have no doubt that the crystals exhibit the deformation described by the authors. However, I have a difficult time judging the novelty and impact of the work. The manuscript is quite applied without much decision of scientific mechanisms, material design considerations, or quantitative analysis. I believe the authors should address a few specific points: 1. The materials should be described in much more detail. I do not believe referring to the crystals as "1" or "2" is appropriate. What are the chemical names or formula of the compounds? 2. Do the authors have any materials characterization related to the actual materials? Have more than one of the same composition of crystal been tested? Is there any relationship between structure/quality of the crystals and the photo-mechanical response?

Response:
We thank the Reviewer for the insightful comments. We acknowledge the Reviewers' concerns and the comments related to the advantages of photothermal methods. In our detailed response to their comments below we provide explanation on the importance and advantages of using the photothermal effect and the reason for setting of the direction of bending on the fiber. Comment: 1. The authors used the photothermal effect to bend flexible organic crystals. However, there is a lack of a unique advantage of using photothermal effects. Please provide a unique advantage of using photothermal effects on flexible organic crystals.

Response:
We thank the Reviewer for pointing this out. The photothermal effect has been studied extensively in case of photoactive polymers, such as, for example, liquid-crystalline elastomers, where it has been used to induce rapid deformation. Recently, it was also reported for single crystals or organic molecules. Here, for the first time, we report the effect of photothermal heating on hybrid single crystals. There are several advantages of using the photothermal effect to actuate crystals, one of which is the potential of using light to actuate even crystals which are not photochemically reactive. This is significant advantage which expands the application to any organic crystal. The second advantage is the high speed of deformation which can be achieved by using the thermal effects. The third advantage is the opportunity for remote control of the shape of the crystal, similar to other photoinduced mechanical effects, however in case of photothermal effects this can be performed over very long distances. Finally, the fourth advantage is that by using photothermal effect, one can bend the crystal at specific, predetermined locations, that is, it is possible to have a spatial control over the crystal's deformation. To highlight these points, we modified the abstract, the introduction, and some of the main text of the manuscript.
The following text was added to the introduction section: "The photothermal actuation of the hybrid crystals reported here comes with multiple advantages over other methods that have been used for crystal deformation. Since the absorber is the MXene, this approach does not require absorption of radiation by the crystal itself, which circumvents the necessity for the crystal to absorb light (i.e. be photoreactive). Second, both high speeds and high deformations can be achieved, and the crystal can be actuated by localized thermal excitation at a predetermined location. Finally, we demonstrate that hybrid crystals can be precisely actuated by excitation over very long distances (> 3 meters), which brings an added value to the potentials for real-world applications of dynamic responsiveness of organic crystals, where they could be used as receivers for remote sensing, triggering, or actuation." The following sentences were added to the abstract (one sentence was deleted to shorten the abstract and keep it within the limits): "Photothermal actuation of the organic-MXene crystals circumvents the limiting requirement for photoreactive groups in the organic crystal, and can be applied to any flexible crystal. The method provides high deformation speeds with precisely controlled, localized deformation." "We also demonstrate that excitation can be achieved over very long distances (> 3 meters), which places these materials into the realm of long-distance receivers for remote sensing, triggering, or actuation." The following text has been revised at the end of the conclusions section: Original text: "Adding the IR light to the palette of available excitation stimuli such as light, humidity and magnetic field expands significantly the prospects for construction of flexible optical and electronic devices based on organic crystals." Revised text: "Since photothermal bending has unique advantages over other modes of excitation, such as the possibility to control the point of bending by choosing the position of excitation and to deform the crystal remotely by irradiation over long distances, adding the IR light to the palette of available excitation stimuli such as light, humidity, and magnetic field expands significantly the prospects for construction of flexible optical and electronic devices based on organic crystals." Some of the above advantages are clearly illustrated in our manuscript. For example, as shown in Figure 4a and 5d, flexible organic crystals can be bent at different positions along the crystal, and this can be achieved by controlling the position of irradiation with infrared light. By using the photothermal effect, bending of flexible organic crystal can also be achieved by irradiation over long distances. To emphasize this latter point, Supplementary Movie 9 has been modified, where we show an excitation of the crystal over a distance of 3 meters. Accordingly, the following text have been revised: Original text: "In addition, we established that the actuation of the hybrid organic crystal bending can be controlled by infrared light over long distances (>2 m), as shown in the Supplementary Movie 9, a result which highlights the prospects for long-range sensing or other remote applications of these materials" Revised text: "In addition, as shown in the Supplementary Movie 10, we established that the actuation of the hybrid organic crystal bending can be precisely controlled by infrared light at distances of over 3 meters (Supplementary Figure 20). This result further highlights the prospects for long-range sensing, triggering, actuation, or other remote real-world applications of these materials." Added text: "In the hybrid crystal, the MXene functions as a photothermal converter, while the PVA/PSS layer functions as one of the two components of a bilayer strip that generates bendign moment by expansion or contraction. PVA is a common hygroscopic polymer that has a low critical solubility temperature, and is well known to undergo reversible swelling via hydrogen bond formation (Supplementary Figure 6). 43 This bring about to ressponse of the hybrid element to humidity, and the curvature could change with variation in humidity (Supplementary Figure  7)."  Figure 3, the authors showed the rotation of crystals by changing the position of the glass tube. However, there is a lack of information on how the rotation was possible. Additionally, I believe that, to prove the controllability of the crystals, the authors should fix the glass tube and change the incident light.

Response:
We are grateful for this comment, which brings the necessity for some clarification of the experiment. We apologize that the details of rotation of the capillary glass tube to change the bending direction of the hybrid crystal were not clearly described in the original manuscript. The relevant details have now been added to the text. In addition, the experiment of fixing the glass tube and changing the incident light, which was suggested by the Reviewer, was also performed. As shown in Figure R1 below, when the direction of the incident IR light (left and right) is changed, the bending direction of the hybrid crystal does not change. The reason for this is that, in this work, the polymer was deposited on only one of the wide faces of the crystal (as described on page 5, line 132). When the MXene is heated by light, the polymer shrinks, which affects only the deposited face, and this leads to differential strain at the interface, resulting in bending of the crystal. However, if the deposition method of the PVA/PSS film is modified, bending of the crystal in both directions can be achieved when the capillary glass tube is fixed. As shown in the Supplementary Figure 18, the hybrid crystal can indeed be bent in both directions by changing the direction of the incident infrared light. To clarify this point, the following text has been revised (page 7, line 175): Original text: "The position of the infrared lamp (184 mW) was fixed, and turning of the capillary glass tube results in multidirectional bending of the hybrid crystals ( Figure 3a)." Revised text: "As shown in Figure 3a, the position of the infrared lamp (184 mW) was fixed, and the capillary glass tube below the styrofoam base was manually rotated (Figure 2b). As the crystal glued at the tip of the capillary glass tube rotated, it could be bent in different directions." The following text has been added (page 8, line 205):  Figure 4B, in the case of sample #4, there is an abrupt change in the bending angle. Why does this phenomenon happen?

Response:
We are grateful to the Reviewer's patience with the detailed reading, which really helped us provide better clarification of some of the experimental details. In Figure 4B, the abrupt change in the bending angle for sample #4 could be interpreted as follows. Over the bending process, especially when P 3 //4 bends beyond 90°, the IR beam could illuminate multiple positions of the crystal P 3 //4 at the same time, which causes the bending to increase further. This possible and very probable reason for the observation has been explained in the text (page 9, line 210). Figure 4D, the authors showed the mechanical stability results of the crystal by showing the bending angle of the crystals. However, as the authors emphasized the waveguiding performance of the organic crystals, it could increase readers' understanding by showing optical loss data after 100 folds. Additionally, what happens to optical loss after a long period of input light? The period of the light pulse is missing in the manuscript.

Response:
We thank the Reviewer for the suggestions. We concur with the Reviewer that, indeed, the reader's understanding can be enhanced by showing the optical loss data after 100 folds. In response to this suggestion, the optical loss after a long period of input light and the period of the light pulse have been added to the text, and a new Supplementary Figures  24 and 25 were added with this data.
The following text has been revised on page 11, line 270 in the revised manuscript: Original text: "The distance-dependent emission spectra were obtained by irradiating different positions of P 3 //3,5 by using a 355 nm laser and the emission spectra were collected at the other end of the crystal and fitted ( Supplementary Figure 5a-d)." Revised text: "The distance-dependent emission spectra were obtained by irradiating different positions of P 3 //3,5 by using a 355 nm laser (10 Hz, 10 ns) and collecting the emission spectra at the other end of the crystal and fitting the data (Supplementary Figure 23a-d)." Moreover, the following text has been added on page 11, line 276 in the revised manuscript: Added text: "Moreover, the optical loss of the hybrid crystal was measured after 100 bending cycles and a long time of exposure. As shown in Supplementary Figure 24, P 3 //3, the optical loss at 0, 50, 100-fold bending was 0.16373, 0.16988 and 0.17649 dB mm -1 respectively. The optical loss after irradiation of 0, 30 and 60 minutes was 0.15600, 0.16238, and 0.17458 dB mm -1 , respectively (Supplementary Figure 25)." Figure 24. Dependence of the optical loss on the crystal bending cycles. (a-c) Fluorescence spectra were collected at the fixed end of the crystal, while the crystals were excited at different position by 355 nm laser (10 Hz, 10 ns). The difference in position between the fixed end and the excitation position is defined as distance (mm). Panels a, b, and c correspond to optical loss after 0, 50, 100-fold bending, respectively. (d-f) Decay of intensity with distance Itip/Ibody. The optical loss coefficient (α) was obtained by a single exponential fitting function Itip/Ibody = Aexp(-αD), where Itip and Ibody are the fluorescence intensities measured at the fixed end and the excitation position, respectively. A is the optical loss coefficient and D is the distance between the fixed end and the excitation position. The panels show P 3 //3 after 0 (d), 50 (e), and 100 (f) bending cycles. Figure 25. Dependence of the optical loss on the duration of excitation. (a-c) Fluorescence spectra were collected at the fixed end of the crystal, while the crystals were excited at different position by a 355 nm laser (10 Hz, 10 ns). The difference in position between the fixed end and the excitation point is defined as distance (mm). Panels a, b, and c correspond to optical loss after excitation of 0 min, 30 min, and 60 min, respectively.

Response to the comments from Reviewer #2:
General comments: In this article, the authors explored the flexible elastic crystals for optical waveguides. The needle-shaped crystals were converted into hybrid materials by coating them with layers of polymers and MXene. This hybrid material is explored for IR light-driven flexible material for optical waveguides, where the hybrid crystal array responds to the duel stimuli differently (IR and UV light). This is indeed one step ahead in the progress of crystal engineering and its application in various optoelectronic devices.
The authors should address the following two comments before publication.

Response:
We are grateful to the Reviewer for their constructive and encouraging comments, and the overall positive assessment of our manuscript. Below, we provide a point-by-point response to their comments and suggestions.

Comment: 1. From the discussion and methodology, it is not very clear how the hybrid crystals are created exactly. More details procedures are required for the coating experiments.
Response: In response to the Reviewer's comment, additional details on the procedure used for preparation of the hybrid crystals and the coating were provided in the revised version of the manuscript. Specifically, the following text was revised and expanded (page 5, line 123), and a new Supplementary Figure 5 was added to the supplementary materials that illustrates the process of preparation of the hybrid crystals.

Response:
We thank the Reviewer for the suggestion. In response to the Reviewer's comment, a discussion on the mechanisms of bending and waveguiding have been added to the text.
The following text has been added on page 5, line 132 in the revised manuscript: Added text: "Since only one of the two wide faces of the crystal were coated with polymer, when the polymer shrinks, it gives rise to a differential strain that translates into a bending moment. This is observed as macroscopic bending of the hybrid crystal. In the hybrid crystal, the MXene functions as a photothermal converter, while the PVA/PSS layer functions as one of the two components of a bilayer strip that generates bending moment by expansion or contraction. PVA is a common hygroscopic polymer that has a low critical solubility temperature, and is well known to undergo reversible swelling via hydrogen bond formation (Supplementary Figure 6). 43 This bring about to response of the hybrid element to humidity, and the curvature could change with variation in humidity (Supplementary Figure 7)." New Supplementary Figure 6. Mechanism driving the bending by photothermal effect of the hybrid crystals. The diagram shows swelling or contraction of the polymer layer induced by heating by the MXene induced by infrared light.

Response to the comments from Reviewer #3:
General comments: This is an interesting piece of work. The application of MXene-based materials in the field of flexible optical waveguide is a novel subject. This work provides a simple and effective way to realize the flexibility of optical waveguide on organic crystals, and this method is applicable to a variety of materials. Moreover, 2D array with high sensitivity, high and controllable degree of deformation and durability over prolonged actuation is also carried out, while the bending point of each crystal can be controlled by changing the infrared light position. Therefore I recommend this article can be considered for publication after the authors have addressed the following concerns: Response: We thank to the Reviewer, who apparently is a knowledgeable expert in this field, for recognizing the significance and the impact of the work presented in our manuscript, as well as for their generally positive assessment. In the revised version of the manuscript, we provide response to each of the points they have raised, as well as a description of the changes made.
Comment: 1. The bending of organic crystals caused by thermal effect should be further analyzed theoretically. Does it involve physical changes such as phase transition or chemical changes?

Response:
We are grateful to the Reviewer for their careful inspection of the details. The bending is generally a physical phenomenon caused the generation of differential strain between two conjoined elements that have different propensity for expansion, similar to a bilayer strip. This expansion (or contraction) can be cause by either physical means, such as thermal response of the material, or chemical process such as isomerization, cyclization, etc. In either case, the result is change in length of one of the components in respect of the other component. In order to ascertain whether any chemical changes have occurred in our samples during excitation, we performed NMR analysis on the original crystals 2-4 and on the crystals 2-4 after heating them at 100 ºC for 1 hour. Moreover, we performed differential scanning calorimetric (DSC) analysis on crystals 2-4. The results confirmed that the crystals did not undergo phase transitions or chemical changes during irradiation. This data are now provided in the revised version of the manuscript.
The following text has been added on page 5, line 142 in the revised manuscript: Added text: "To further confirm that the bending of the hybrid crystals was a result of thermally induced mechanical process and not a physical phase transition or a chemical reaction, NMR analysis of 2-4 was performed before and after heating at 100 ºC for 1 hour, and the compounds were also analyzed by differential scanning calorimetry (DSC). The results ruled out phase transitions or permanent chemical changes." Response: In order to verify this point, hybrid crystals having different sizes of the organic crystal, as well as such having same crystal size but with different thicknesses of MXene were prepared and studied. The experimental results confirmed that sample size and deposition thickness do, indeed, affect the performance of the 2D array. The new experimental results have been added to the revised version of the manuscipt.
The following text has been added on page 3, line 98 in the revised manuscript: The following text has been added on page 7, line 181 in the revised manuscript: Added text: "Hybrid crystals with organic crystals having different size as well as with organic crystals of the same size but having different thickness of the MXene layer were also prepared to examine the effect of crystal size and MXene thickness on the performance of the 2D array. As shown in Supplementary Figure 16